Sqeaking Shifting

So, when I'm in the big ring on my road bike, and I **** UP a cog in the back, I hear a terrible screech. This ONLY happens in the big ring, and ONLY shifting into larger (lower) cogs. It is definitely come from the rear deralleur. Any ideas on what this might be, and how to fix it?

So, when I'm in the big ring on my road bike, and I **** UP a cog in the back, I hear a terrible screech. This ONLY happens in the big ring, and ONLY shifting into larger (lower) cogs. It is definitely come from the rear deralleur. Any ideas on what this might be, and how to fix it?

Try lubing the 2 tension pullys on the rear derailleur with Triflow. If that doesn't work lube every pivot on the rear derailleur (like you should for regular maintenance, anyway). If that doesn't work, it might be somthing rubbing on your front derailleur. Go to you LBS and ask for an opinion.